Mysteries of Capotauro: investigating the puzzling nature of an extreme F356W-dropout

TL;DR

Capotauro, a faint F356W-dropout in CEERS, is analyzed with JWST NIRCam, MIRI, and NIRSpec CAPERS data plus HST imaging to determine whether it is a $z\geq15$ galaxy, a $z<10$ dusty Balmer-break interloper, a strong-line emitter, or a Milky Way substellar object. The study employs three SED-fitting codes (BAGPIPES, CIGALE, ZPHOT) and cool brown-dwarf atmosphere templates (Sonora Cholla, ATMO) to explore extragalactic and Galactic scenarios, including open redshift priors up to $z\leq40$. Across codes, the most robust extragalactic interpretation is a bright Lyman-break galaxy at $z\sim32$, with $M_{UV}\sim-21.5$ and only roughly $0.5\%$ of the posterior volume at $z<25$; a potential $z\sim29$ solution could arise if a tentative $3.63\,\mu$m feature is Ly$\alpha$. However, a very cold brown dwarf or a free-floating planet with $T_{\rm eff}\lesssim300$ K at tens to thousands of parsecs remains viable within the data. Given the current data, Capotauro remains a remarkably unique object, and its nature will only be resolved with deeper follow-up spectroscopy and mid-IR observations, carrying implications for early galaxy formation and the census of cold substellar objects.

Abstract

JWST has uncovered a diverse population of extreme near-infrared dropouts, including ultra high-redshift ($z>15$) galaxy candidates, dust-obscured galaxies challenging dust production theories, sources with strong Balmer breaks - possibly compact AGN in dense environments - and cold, sub-stellar Galactic objects. This work presents Capotauro, a F356W-dropout in the CEERS survey with F444W AB magnitude of $\sim27.68$ and a sharp $>3$ mag flux drop between $3.5{-}4.5\,μ$m, undetected below $3.5\,μ$m. We combine JWST/NIRCam, MIRI, and NIRSpec/MSA data with HST/ACS and WFC3 observations to perform a spectro-photometric analysis of Capotauro using multiple SED-fitting codes. Our setup tests $z\geq15$ as well as $z<10$ dusty, Balmer-break or strong-line galaxy solutions, and the possibility of Capotauro being a Milky Way sub-stellar object. Among extragalactic options, our analysis favors interpreting the sharp drop as a Lyman break at $z\sim32$, consistent with the epoch of formation of the first stars and black holes, with only $\sim0.5\%$ of the posterior volume at $z<25$. Lower-redshift solutions struggle to reproduce the extreme break, suggesting that if Capotauro lies at $z<10$, it must show a non-standard combination of strong dust attenuation and/or Balmer breaks, making it a peculiar interloper. Alternatively, its properties match a very cold (Y2-Y3 type) brown dwarf or a free-floating exoplanet with a record-breaking combination of low temperature and large distance ($T_{\mathrm{eff}}<300\,\mathrm{K}$, $d\gtrsim130\,\mathrm{pc}$, up to $\sim2\,\mathrm{kpc}$). While current data cannot determine its nature, Capotauro emerges as a remarkably unique object in all plausible scenarios, and a compelling target for follow-up.

Figures (11)

• Figure 1: Top panel:Capotauro's complete JWST and HST photometry in AB magnitudes (left plot), with detections marked as circles and 1$\sigma$ upper limits as triangles. On the right we report Capotauro's JWST/NIRCam F356, F410M and F444W 1.2$\,\times\,$1.2 cutouts (full multi-band single and stacked cutouts are available in Figure \ref{['fig:cutouts']}). Bottom panel:Capotauro's 2D JWST/NIRSpec CAPERS spectrum as a function of the observed-frame wavelength. The $\lambda\!\sim\!3.63\,\mu$m potential emission feature is highlighted by a white circle. Below, we report the extracted CAPERS spectrum (dark gray line) with 1$\sigma$ errors (light gray shaded area). The red curve denotes the spectrum rebinned into wavelength bins of width $\Delta \lambda\!=\!0.1\,\mu$m. The surrounding red shaded band illustrates the standard error of the mean in each bin, while the black dashed line marks the zero flux level. The inset of the figure displays all the available NIRspec PRISM slits of our CAPERS observations (red for the ones not containing the source, which were rejected, and green for the ones containing the source).
• Figure 2: Top Left: Radial profile of Capotauro re-centered to its brightest pixel (black line) compared to the median profile of 89 empirical PSF sources injected in the F444W image and flux-matched to Capotauro (in blue, with a shaded area representing the 25–75 percentile range of the 89 profiles). An empirical CEERS F444W PSF profile (red line) is also displayed, as well as a spectroscopically confirmed F444W-detected BD in CEERS (green dashed line; JWST/NIRSpec MSA ID 1558; 2024MNRAS.529.1067H). Uncertainties are calculated as the standard error on the mean flux within concentric 1-px annuli. Top Right: Lower limit on Capotauro's break strength (dashed orange line) compared to the one of the Sonora Cholla cloudless BD atmosphere models (circles) for different effective temperatures, color-coded by their surface gravity value (i.e., $\log{g}$). Bottom: Rescaled F356W fluxes of the 41 $\chi^2_{\rm red}<2$ Sonora Cholla BD templates versus their effective temperature, color-mapped by their surface gravitational acceleration. These templates are compared to Capotauro's $1\sigma$ upper limit (orange dashed line). For reference, we provide Capotauro's ATMO BD template best-fit temperature (T=300 K), represented as a black dotted line.
• Figure 3: Capotauro's CAPERS spectrum (gray line) and rebinned spectrum (see Figure \ref{['fig:capers_spectrum']}) with HST and JWST/NIRCam photometry (black circles, with 1$\sigma$ upper limits depicted as triangles) compared to our best-fit ATMO 2020 template (2020AA...637A..38P; blue line). For comparison, we report the JWST/NIRSpec PRISM spectrum of the coldest known BD WISE J0855-0714 (2024AJ....167....5L; green line) and of the Y0 BD WISE 0359-5401 (2023ApJ...951L..48B; orange line) rescaled to match Capotauro's JWST/NIRCam photometry (from $d\!\sim\!2.28\,$pc to $d\!\sim\!307\,$pc and from $d\!\sim\!13.6\,$pc to $d\!\sim\!1.1\,$kpc respectively).
• Figure 4: Upper limit on Capotauro's proper motion (black dashed line) compared to the one of other known Y BDs as function of their distance (colored circles). Colored lines show the relation $\mu = v_{\rm tan}/(4.74\,d)$ for different galactic components: thin disk ($v_{\rm tan}\!\leq\!50$ km s$^{-1}$, solid), thick disk ($50\!<\!v_{\rm tan}\!\leq\!200$ km s$^{-1}$, dashed), and halo ($v_{\rm tan}\!>\!200$ km s$^{-1}$, dotted). The color scale indicates different values of $v_{\rm tan}$. The gray shaded area represents distance values excluded by the best-fit flux normalization.
• Figure 5: Left panel: Results of our open-redshift SED-fitting runs based on Capotauro's photometry. Observed fluxes (black dots; 1$\sigma$ limits as triangles) are compared to best-fit SEDs from BAGPIPES (red), CIGALE with a Dale14 AGN component (yellow), and ZPHOT (purple). Red diamonds indicate synthetic photometry from the BAGPIPES fit. The gray line shows Capotauro's spectrum from the CAPERS survey. The inset shows the P($z$) distributions from all three codes (linear scale). Right panel: Normalized logarithmic redshift probability distribution yielded by BAGPIPES (in red), CIGALE (in yellow) and ZPHOT (in purple). The black dashed line represents the average best-fit redshift between our BAGPIPES, CIGALE and ZPHOT runs. We report redshift values of secondary P(z) peaks for CIGALE and ZPHOT.